@saphirekosmos He did have had time to mentally prepare himself, since he was on a rescue mission for what he believed was her period.
But still, well done by him, twice even. (First when he set out just because he understood that it might be important, and then when he like you said managed to take the further discussion in stride.)
If anything, she's more stressed out that he might think she's a heavy bleeder.
(Or basically, she's worried that he might be grossed out, like boys might be)